Variable-length antenna



VARIABLE-LENGTH ANTENNA Filed Jan. 12, 1958 2 Sheets-$heet 1 INVENTOR WALTER BER/V07 ATTORNEY Aug. 8, 1939. w. BERNDT 2,168,860

VARIABLE-LENGTH ANTENNA Filed Jan. 12, 1938 2 Sheets-Sheet 2 4 Amax. /6

INVENTOR WALTER BERNDT BY HQW ATTORNEY Patented Aug. 8 1939 VARIABLE-LENGTH ANTENNA Walter Berndt, Berlin, Germany, assignor to 'Telefunken Gesellschaftfiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application January 12,1938, SerialNo. 184,534

In Germany December 16, 1936 6 Claims. ((1250533) I x It known fromthe earlier art thatantennae which are to operate within and cover a large wave-band must be constructed so that the length thereof may be altered. It is also knownin the prior artthat the antenna may be tuned by steady variation of the antenna length so that adaptation or matching, throughout the entire band or range, to anenergy feeder lead or transmission line will be insured. In order that alter- 10 ation in the length of the antenna may be effected mechanically the shifting means must be insulated for the radio frequency potential between ground and antenna or inner conductor of the feeder cable. Where large transmitter powll ers and the high potentials connected therewith.

are concerned, this problem is attended with serious technical difliculties from an insulation viewpoint.

The suggestion is therefore made according to 20 this invention to leave the rod-shaped antenna conductor directly connected with the inner conductor of the concentric or co-axial feeder line itself unaltered, while instead the counterpoise which is connected with the end of the outer conductor of the energy transmission line is made capable of beingshifted, with the result that the effective or active part of the antenna protruding from the outer conductor is varied in its length. This scheme has the advantage that such means 80 as are used for alteration of the length, say, motors, air-pressure cylinders, or the like, are at ground potential and may operate without any insulating intermediary parts. Furthermore, also, such devices as are provided for the purpose ll of indicating the effective antenna length to which the aerial may have been adjusted at a remote point, will always be at ground potential so that they may be readily shielded against effects of the radio frequency potential. In this scheme, (a the counterpoise fulfills two important functions, to wit: it allows the distribution of the field of an antenna erected above'an infinitely large conducting surface to be as uniform as feasible; and it prevents induction currents (and their 45 harmful effects) from arising over the radiation field at the antenna end of the concentric or co-axial feeder or transmission line.

If the counterpoise were to be dispensed with,

accurate matching with a relatively large wave to band would no longer be insurable.

An illustrative embodiment of the idea underlying the present invention is shown in the drawing Fig. 1. Fig. 2 shows a modificationfor a guyed antenna; Fig. 3 shows a mechanical ar- 55 rangement for shifting the height of the counterpoise; and Figs. 4 and 5 show a further modification of the invention in two different operative positions.

' Referring to Fig. 1, A denotes the antenna, G is the shiftable counterpoise which consists of a 5 disk with a piece of pipe attached thereto. The disk may consist of sheet-metal, wire mesh or wires or ribbons extending and fastened radially waster-fashion. The disk may be of any shape,

but it is preferable that the top face of the disk should consist of a material possessing high electric conductivity.

In the embodiment shown irrFig. 2, the counterpoise G has notches which serve to accommodate the guy wires or cables P. It is thus feasible to shift the counterpoise to a point in close proximity to the points where the stay cables or wires are attached.

Fig. 3 shows the one means of causing the displacement or shifting of the counterpoise. Also in this figure, G denotes the counterpoise which is shiftable on' the insulator piece I on the antenna A. K. is a high-frequency cable with the terminal box or closure member E, the inner conductor, thereof being connected with the antenna A. Current is fed from the cable to the counterpoise by way of the tubular or pipe piece S.

The tubular part of the counterpoise G conjointly with the extension of 1 the antenna constitutes the energy feeder whose characteristic impedance is governed by the ratio of the two diameters. Choosing the same diameter ratio as for cable K, then the cable is electrically extended as far as the antenna base. If the antenna has a length so that the base resistance is ohmic in nature (approximately M4, M2, 3/4), etc. where (x) denotes the operating wave lengt and if the characteristic impedance of the cab e is made equal to the resistance at the antenna base, it will be possible to obtain accurate matching within a wide wave-band. The size of this wave-band depends only upon the extent to which shifting of the counterpoise is mechanically feasible.

It is not possible in all cases to make the characteristic impedance of the cable equal to the resistance of the antenna base or terminal. For reasons of safety against shock, commercial radio frequency cables have a characteristic impedance of around 60 ohms. In such a case it is expedientto so choose the characteristic impedance of the pipe piece interposed between antenna and cable that it ranges between the value of the resistance at the antenna base and the characteristic impedance of the cable. As a limit, the same could be made equal also to either of the said two resistance values.- It the resistance of the antenna. base and the charac- -teristic impedance of the cable are not unduly tials whenever extremely large transmitter powers are dealt with, the desired end condition may be obtained by the insertion of several pipe pieces of dissimilar characteristic impedance values between the antenna and the cable.

An exemplified embodiment of this idea is illustrated in Figs. 4 and 5. Referring to these figures, G again stands for the counterpoise disposed so as to be shiftable along the aerial A, the counterpoise being supported upon the in-- sulator ring I and the current contact piece S. The embodiments Figs. 4 and 5 differ from each other only by the length of the shiitable counterpoise. The antenna itself and its extension consist of two pipesof diflerent diameters. The length is made about equal to of the maximum working wave length. The counterpoise is connected with two tubes which are also of dissimilar diameters; and its length is also approximately equal to one-fourth the maximum working wave length. It will thus be seen that between the antenna base point and the cable terminal box there are arranged three tubular pieces of diflerent characteristic impedance, and all of them are caused to have their lengths altered as a result of a shift of the counterpcise. The top and the bottom pipe piece become greater in length when the counterpoise is raised, while the central pipe piece is shortened by exactly the same amount by which the length of the antenna is abbreviated. For instance, if the length of the antenna is )l/4, then also the central pipe piece will always be equal to i/4 no matter what the working wave. This fact may be utilized for the purpose of obtaining perfectly automatic matching to the cable, even under conditions where the antenna base resistance and the characteristic impedance of the cable differ markedly from each other. To this end the characteristic impedance of the upper pipe piece as is made equal to the antenna base resistance. In other words, at the bottom end the resistance is also :30, that is, there is perfect matching for this pipe piece. The characteristic impedance of the bottom pipe piece at is made equal to the characteristic impedance of the cable 2x. By means oi the central piece of pipe having a length equal to A/, resistance 20 can be adapted to 2 if the characteristic impedance 2m 01' the middle pipe piece is chosen in accordancewith the known relation z m=Zo'Zu. The characteristic impedance of the middle pipe piece zm must therefore range between value 20 and value 2:, and preferably it should be made equal to the geometric mean of the two values. Under this condition, perfectly automatic matching of the antenna to the cable is realized independently of the wave-length.

I claim:

1. A variable length antenna comprising a radiating rod, a tubular member surrounding a portion of said rod and having a counterpoise disc attached to one end, a concentric cable transmission line having its outer sheath slidably connected to said tubular member and its center conductor connected to said radiating rod.

2. A vertical antenna comprising a radiating rod, a tubular member surrounding a portion 01 said rod and having a counterpoise disc attached to its upper end, a concentric cable transmission line having its outer sheath slidably connected to said tubular member and its center conductor connected to said radiating rod. I

3. A vertical antenna comprising a radiating rod, a tubular member surrounding a portion of said rod and having a counterpoise disc attached to its upper end, a concentric cable transmission line having its outer sheath slidably connected to said tubular member and its center conductor connected to said radiating rod, stay wires supporting said rod in its vertical position and notches in said disc for accommodating said wires.

4. A vertical antenna comprising a radiating rod, a tubular member surrounding a portion of said rod and having a counterpoise disc attached to its upper end, a concentric cable transmission line having its outer sheath slidably connected to said tubular member and its center conductor connected to said radiating rod, and means for shifting said tubular member and the disc attached thereto whereby the free length of said radiating rod may be varied.

5. An antenna system comprising a radiating rod having a length equal to one-quarter of the maximum wave length with which it is desired to operate said antenna, a concentric cable transmission line having a central conductor and an outer sheath, an intermediate conductor having the same length as said radiating rod and interposed between said rod and the central conductor of said transmission line, a tubular member surrounding a portion of said rod adjacent said in termediate conductor, said intermediate conductor and a portion of said transmission line; means for connecting the sheath of said transmission line to said tubular member and a counterpoise disc attached to said tubular member at the end adjacent said radiating rod, said tubular member and counterpoise disc being shiftable longitudinally along said radiating rod whereby the operating wave length may be adiusted.

6. An antenna system comprising a radiating rod having a length equal to one-quarter of the maximum wave length with which it is desired to operate said antenna, a concentric cable transmission line having a central conductor and an outer sheath, an intermediate conductor having the same length as said radiating rod and interposed between said rod and the central conduotor of said transmission line, a tubular member surrounding a portion of said rod adjacent.v

said intermediate conductor, said intermediate conductor and a portion of said transmission line, means for connecting the sheath of said transmission line to said tubular member and a counterpoise disc attached to said tubular member at the end adjacent said radiating rod, the diameter of said tubular member at the end adjacent said radiating rod being so chosen with respect to said rod that the impedance of the transmission line formed by said tubular member and the portion. of said radiating rod therewithin is equal to the radiation resistance of the tree portion of said radiating rod, the diameter 0! said intermediate conductor being so chosen aaeaeec 3 with respect to the diameter of said tubular member that the impedance of said intermedi-- ate conductor is equal to the geometric mean between the radiation resistance of said radiating rod andthe impedance of said transmission line and the diameter of the end of said tubular member remote from said counterpoise disc beingso chosen with respect to the diameter of said intermediate member that the characteristic impedance is equal to the impedance of the transmission line.

' WALTER BERNDT. 

